The present invention relates to a process and agent for bleaching wood pulp. More particularly, the present invention is concerned with adjuvants for bleaching thermorefined wood pulp (TMP), chemothermorefined wood pulp (CTMP), mechanical wood pulp, cellulose and waste paper by means of peroxides or salts of dithionic acid and derivatives thereof and of sulphurous acid, as well as derivatives thereof.
Bleaching of wood pulp which is necessary for the use thereof in the production of paper generally takes place with the help of an oxidative or reductive bleaching. The bleaching can be carried out in one or two steps.
The bleaching agent is influenced by the kind of wood, the wood pretreatment, the amount of bleaching agent, the quality of the water, the manner of dosing it in and the nature of the additives used. The average increase of the degree of whiteness is from 8 to 10 points in the case of a one-step bleaching and is from 16 to 18 points in the case of a two-step bleaching.
It is known that the stabilization of the bleaching solution necessary for the achievement of the desired degree of whiteness is, in the case of hydrogen peroxide, carried out with the help of waterglass. However, the use of waterglass leads to processing problems which are brought about, for example, by depositions in the machine parts used and difficulties in the clarification of waste water. For the reduction of these problems, it is necessary to use an increased amount of chemical adjuvants which, in turn, gives rise to new problems when, as is known, the process is carried out in a cycle.
Furthermore, the dosing of waterglass is problematic, the reason for which, in turn, is due to the quality of the water used. Hitherto, it has not been possible to make available products which permit a complete replacement of waterglass in the bleaching of wood pulp, not only in the case of oxidative but also in the case of reductive bleaching.